I remember visiting Colorado in the ‘90s (had been there long enough to acclimatize to the altitude) and had driven up as high as over 12,000 feet and spent the night in such places as Leadville & Cripple Creek, both over 10,000 feet. After leaving the latter place, we drove down to Colorado Springs, which is a “lowly” 6,000 feet. Stopping at “Garden of the Gods”, we encountered a busload of New York City firefighters, all of whom were in good shape physically, but huffing and puffing just merely strolling across a small parking lot like they were out-of-shape fatties. I wasn’t exactly the model of health then, but I was snickering that I was (at least at that moment) better able to handle the high altitude than all these strapping guys.
I have been going to Colorado and New Mexico every few years since I was a child. It was only on 2 of those dozens of trips that the altitude affected my heart. I might be a little out of breath or feel like my feet were heavy at 10,000 feet but never had my heart act like it did on those 2 trips. The most recent was about 5 years ago and then on a trip 10 years before that. This last time I should have gone to the ER, I’ll bet they would have given me a portable oxygen tank.